The Federal Communications Commission and cable television testing organizations such as CableLabs have been evaluating digital television delivery systems in order to choose a new television "standard" which someday will replace NTSC in the United States. These systems all involve digital coding and data compression techniques, for example those utilizing the MPEG digital coding algorithms or variations thereof.
The FCC plans to test and approve an advanced television (ATV) standard comprising for example, high definition television (HDTV) and standard definition (SDTV) digital signals for terrestrial broadcasting in 1994. Although the specifics of the standard are yet to be fully tested and agreed upon, the FCC has indicated that the system will initially take the form of a so called "simulcast" approach. The new ATV signals will have to fit into currently unused television channels (so-called "taboo" channels) and initially co-exist with conventional analog television signals without co-channel interference.
NTSC will be used hereinafter to represent one example of conventional television broadcasting. Other examples would be SECAM and PAL. Although NTSC is exemplified herein, it is not meant to be construed as a limitation and will be used herein synonymously with "conventional" to represent conventional television in general.
In 1994 the FCC will test the so-called "Grand Alliance" system, a system which is being cooperatively developed by the corporate sponsors which developed the first round of individual proposals which were tested by the FCC in 1991 and 1992. This system proposes to take the best features from those systems already tested in order to present a single optimum system for FCC approval as the U.S. standard.
The Grand Alliance has already decided on a coding algorithm which will comply with the source coding standards proposed by MPEG (Motion Pictures Experts Group). In addition, two RF transmission schemes will be evaluated for best performance at a "bakeoff" at the Advanced Television Test Center (ATTC) and one will be selected for inclusion in the Grand Alliance system.
The first system, which was proposed by the American Television Alliance comprising of Massachusetts Institute of Technology (MIT) and General Instrument (GI) Corporation, is described in "Channel Compatible Digicipher HDTV System", Apr. 3, 1992 and also in "Digicipher HDTV System Description", Aug. 22, 1991 which are incorporated by reference herein. This system has been further modified for the bakeoff, the details of which are described in "Summary Description of Terrestrial and Cable Bakeoff Systems" Jan. 17, 1994, which is incorporated by reference herein. This system features the use of quadrature amplitude modulation (QAM).
The second system, which was developed by Grand Alliance member Zenith Electronics Corporation, utilizes a multi-level vestigial sideband (VSB) modulation approach which is described in "Digital Spectrum Compatible--Technical Details" Sep. 23, 1991 and which was recently modified for the bakeoff and described in "VSB Transmission System: Technical Details", Dec. 17, 1993. Both of these publications are incorporated by reference herein.
A third system was proposed by the Advanced Television Research Consortium (ATRC), which included Grand Alliance members Philips Electronics, North America Corporation, David Sarnoff Research Laboratories and Thomson Consumer Electronics, and described in "Advanced Digital High Definition Television--System Description", Jan. 20, 1992 which is incorporated by reference herein. The ATRC system also featured the use of QAM.
All of these systems proposed different methods to alleviate the degradation in performance of the "simulcast" ATV transmission system caused by the co-channel conventional television transmission.
In the GI/MIT approach, the receiver equalizer is made very large to compensate for the co-channel conventional television interference.
In the Zenith approach, a comb filter is used in the receiver to introduce nulls in the digital spectrum at the locations of the co-channel (e.g. NTSC) picture, color and the sound carriers. This provides a significant improvement in performance when conventional television, e.g. NTSC, is broadcast on a co-channel.
The ATRC system introduces a null in the transmission spectra of the HDTV signal at the picture carrier by transmitting QAM over two separate carriers which are frequency division multiplexed into frequencies above and below the picture carrier respectively.
From the test results on these systems, it was observed that the ATRC system performed best when co-channel NTSC, interference was present. The ATRC system did not perform as well however in the presence of additive white Gaussian noise (AWGN) .
In the initial Zenith approach, a 4 VSB modulation scheme was used. This was then modified to a trellis-coded 8 VSB modulation approach.
In the Zenith 4 VSB approach, the data in the transmitter is pre-coded to eliminate error propagation and related to the post-comb used in the receiver in a unique way, as described in U.S. Pat. Nos. 5,086,340, 5,087,975 and 5,121,203 which are incorporated by reference herein. To reduce co-channel NTSC interference, Zenith uses a comb filter with a delay element of 12 symbol intervals, as described in the '975 patent. For a delay of 12 symbols in the comb filter, it is necessary to have a delay element of exactly 12 symbols in the pre-coder (located at the transmitter) as well. Thus, when co-channel interference is present, the combination of the pre-coder and the comb filter provides good performance.
When co-channel NTSC interference is absent and only AWGN is present however, the use of a comb filter at the ATV receiver causes a loss in error performance of 3 dB. This is because of the structure of the comb filter which has a single delay of 12 symbols adding to the direct path which causes the noise to be added as well. This is discussed in the '340 patent. Hence, when co-channel NTSC interference is absent, an alternate path is provided at the receiver. This alternate path performs a post-coding operation which is simply the inverse of the pre-coding operation at the transmitter. The decision on whether the comb filter path or the post-coder path is selected depends upon the measured error-rate of the periodically sent data field sync symbols at the outputs of both the post-coder and the comb filter paths. Whichever error is smallest at the end of a preset period determines whether the post-coder or comb filter is active. This is described in section 6.3.9 of the reference "Digital Spectrum Compatible--Technical Details".
When trellis coding is introduced with the 8 VSB approach however, it is not possible to implement a similar post-coder path, since "soft-decision" Viterbi decoding is used with trellis coding as described in "Principles of Digital Communication and Coding" authored by A. J. Viterbi and J. K. Omura and published by McGraw Hill in 1979, which is incorporated by reference herein. Hence, use of a comb filter at the receiver will always result in a 3 dB loss, outweighing any advantages that the trellis-coding may provide in terms of improved performance for an AWGN channel.
The solution proposed by Zenith for trellis coder VSB is to remove the pre-coder at the transmitter, and then treat the comb filter at the receiver as a partial response channel in cascade with the trellis coder when co-channel conventional television interference is present. An optimum decoder can then be developed which uses Viterbi decoding on an expanded trellis, the states of which correspond to the cascade of the states of the comb filter and the trellis coder as described in "Principles of Digital Communication and Coding".
For a comb filter with a delay of 12 symbols however, the number of trellis states is extremely large. To simplify their design, Zenith first converts the MPEG coded and RS (Reed-Solomon) coded and interleaved serial data-stream to a parallel data stream and then uses 12 parallel trellis encoders followed by a parallel-to-serial converter at the transmitter. The trellis decoder implements Viterbi decoding on a trellis when the post-comb filter is used, with the number of states being equal to two or four times the number of states of the trellis encoder. This is described in detail in "VSB Transmission System: Technical Details".
When co-channel conventional television interference is absent, Viterbi decoding is implemented on a trellis with the number of states equal to the number of states of the trellis encoder. This is possible since pre-coding is not used in the transmitter. As in the 4 VSB case, the choice between the path afforded by simple trellis decoding or of that using the post-comb filter and the expanded trellis at the receiver is decided by the measured error-rate of the periodically sent data field sync symbols at the outputs of the post-comb filter and with no post-comb filter.
When both co-channel interference and AWGN are present however, the performance of the comb filter degrades dramatically. This is because the AWGN after the comb filter does not remain "white", but gets "colored", in other words the noise samples are not independent of each other. This affects the performance of the trellis decoder which is optimized for performance in an AWGN channel. Since the co-channel conventional television interference is maximum at the fringe area where the signal power is small and hence the AWGN is large, this is indeed a scenario which must be taken into account. A first object of the instant invention is therefore, to improve the performance of an ATV receiver when co-channel interference and a high AWGN level are present.
The number of states of the trellis encoder is limited by the fact that the Viterbi decoder for the comb-filter path must operate on a trellis with at least double the number of states of the trellis encoder. This limits the AWGN performance of the trellis encoder/decoder when co-channel television interference is not present. A second object of the instant invention therefore, is to improve the AWGN performance of the trellis encoder/decoder in an ATV receiver when co-channel television interference is not present.
Finally, the switching between the use of a comb filter in the receiver or not, suggested by Zenith is cumbersome. A significant number of computations must be performed to determine whether the comb filter should be used or not. Furthermore the use of the comb filter also specifies the use of 12 parallel encoders and correspondingly 12 parallel decoders which also is cumbersome. Another object of the invention therefore is to avoid the use of a comb filter at the receiver.